1. Field of the Invention
The invention relates to systems for transmission of multiple information signals in a band; and more particularly to systems in which a plurality of signal channels are transmitted using respectively different carrier frequencies, and where it is desirable to occupy the entire band so that spectrum utilization is optimized.
2. Description of the Prior Art
Techniques for avoiding interference between different transmitted information streams are very old. The earliest technique involves the use of respectively different carrier frequencies for the different information streams. A tuned amplifier stage discriminates between the desired modulated carrier, and the undesired information streams with which other carrier frequencies have been modulated.
Another technique, exemplified by European Patent Application EP A 0 429 200 A2, involves the use of directionality and difference in frequency sets. Directionality means that certain signals are radiated by a first transmitting antenna array which is highly directional--the signal intended for reception by one customer is radiated in that direction, using an antenna array which has a relatively narrow beam angle, while signals for another customer, whose location relative to the first customer places him outside the first array's coverage, are transmitted by a second antenna whose coverage does not include the first customer. Additional protection against interference comes from using different "frequency sets," where one frequency set differs from another because the frequencies occupied by their channel bands are mutually exclusive, or they differ in direction of polarization.